proposal: short => rewrite for function declarations

[Adam D. Ruppe]

On Saturday, 10 October 2020 at 20:43:55 UTC, Steven 
Schveighoffer wrote:
> 1. is short for:
> (T)(T v) {
>   return v.foo;
> }

This technically isn't true... the lambda expression actually 
does context-dependent magic that this template can't represent 
either.

Like that is *part* of its implementation detail, but there's 
more to it.

So since it is indeed impossible to duplicate its 
context-dependent magic, I made it a parse error:

https://github.com/dlang/dmd/pull/11833/commits/acd15ce776af9bab336c14df20d77fbdc708e368

should be a happy enough compromise.